Back in HS, I was a 5'8" 185 lb starting Center. Needless to say, I worked my ass of in the weight room in order to have a shot to compete in the games against guys that weight at least fifty pounds more than I did. My offseason workouts were pretty much all on me, and I focused primarly on bench press and squats.
My bench workout was usually 3x5 with two pound escalation each set, i.e. 5@185, 5@205, 5@225. Squats were usually a warmup, then three sets of 10 along the same lines of escalation. We also had a machine that basically simulated exploding off the line, so I did a lot of work on that. I threw in some tricep and bicep work, and then usually did leg extensions and hamstring curls, and I did it three days a week.
I ended up being the strongest guy on our team pound for pound cause I was willing to put in the work and to eat and eat and eat to put on keep weight (though I had some extra pudge around me too).
My conditioning work involved running bleachers, sprints, some basic plyo work, and running up and down hills until I puked. I kicked my own ass in high school to be a decent football player, but I really wish I'd had more guidance, because my lifting could have been better.
If I could go back now, I'd incorporate deadlifts, cleans, and rows in there too. I think Chad Waterbury's Total Body Training would be very solid for a high school football player looking to put on some strength, as I feel like it really hits your core on a regular basis. If you're interested, I did the University of Virginia lifting program last month for weeks, and made some good gains. So I can send that over to you if you want it.
As for your coaches, just from my experience, HS coaches, don't always know their stuff too well. A lot of them are just passing along the same things they did when they were in college twenty years ago. You're going to have to do what they want you to regardless, but do some research on your own, and if you can do your own offseason lifting, do it. It'll pay off.